This invention relates to a phase calibration method and apparatus. More particularly, the invention relates to a phase calibration method and apparatus for inserting a calibration signal into a main signal, inputting the resultant signal to a prescribed circuit and calibrating the phase of the main signal based upon a change in the phase of the calibration signal contained in the output signal of the circuit.
In an adaptive array antenna system known in the art, a base station or a mobile station in a communication system is provided with a plurality of antennas and transmitters and the phases and amplitudes of the ratio waves produced by each of the antennas are controlled independently to form a transmit beam pattern in such a manner that the peak of the pattern will point in a prescribed direction.
FIG. 17 is a diagram illustrating the configuration of a base-station adaptive array antenna system. A direction estimating unit 1 receives a signal that has been transmitted from a mobile station and estimates the direction in which the mobile station lies. A transmit beam former 2 controls the phases and amplitudes of radio waves that are output from antenna elements A#1 to A#n of an array antenna 3, thereby forming a transmit beam pattern in such a manner that the peak of the pattern will point in the direction of the mobile station. The transmit beam former 2 includes a weight controller 2a for outputting amplitude and phase adjustment values ωi, φi of the antenna elements A#1 to A#n in such a manner that the peak will point in the direction of the mobile station, and amplitude and phase adjusting units 2b1 to 2bn for adjusting the transmit-signal amplitudes and phases, which are input to the antenna elements A#1 to A#n, by ωi, φi. DA converters 4a to 4n convert signals, which are output from the amplitude and phase adjusting units 2b1 to 2bn, to analog signals. Analog circuits 5a to 5n each have a mixer 6a, local oscillator 6b, filter 6c and high-frequency amplifier 6d. The mixers 6a of the analog circuits 5a to 5n multiply the transmit signals that are output from the DA converters 4a to 4n by the outputs of the local oscillators and up-convert the signal frequencies from baseband to high frequency. The filters 6c pass desired band components, and the frequency amplifiers 6d subject the filter outputs to high-frequency amplification and apply the amplified signals to the corresponding antenna elements A#1 to A#n. In accordance with the adaptive array antenna system of FIG. 17, the transmit beam pattern can be formed in such a manner that the peak points in the direction of the mobile station.
A problem which arises in the adaptive array antenna system of FIG. 17 is a deviation in each branch ascribable to variations in the amplitude and phase of the analog component parts. This is illustrated in FIG. 18. In a case where it is assumed that the weighting coefficients in weighting control have the same values for all branches, the phases after weighting control will all be identical in the digital section. However, when the signals reach the antennas through the analog circuits 5a to 5n after the digital-to-analog conversion, the phases in each of the branches will exhibit a deviation from one branch to the next. Fabricating the analog circuits 5a to 5n so as to eliminate such deviation is not impossible but is impractical. It is believed difficult to fabricate the analog circuits so as to eliminate fluctuations in manufacturing cost and temperature characteristics and fluctuations ascribable to aging. What is required, therefore, is a phase calibrator. FIG. 19 is a diagram illustrating the configuration of an adaptive array antenna system having a phase calibrator. Specifically, a phase calibrator 7 is provided on the output side of the transmit beam former 2 that performs weighting control. The phase calibrator 7 has phase adjusters 7a1 to 7an for adjusting transmit-signal phase input to the antenna elements, and a phase controller 7b for generating phase adjustment values. The phase calibrator 7 corrects for the phase changes in the analog circuits 5a to 5n. That is, the phase calibrator 7 exercises control so as to apply a phase correction having characteristics opposite the phase characteristics of the analog circuits 5a to 5n, as a result of which the phases will agree at the array antenna 3.
In order to perform the calibration shown in FIG. 19, it is necessary to observe the signals that have passed through the analog circuits 5a to 5n and measure the phase characteristics (phase lead/lag characteristics) of the analog circuits. Adaptive array antenna systems each having a structure for this purpose are illustrated in FIGS. 20 and 21.
In order to calibrate phase, it is necessary to insert a calibration signal into the transmit signals of each of the branches in turn, input the resultant signals to the analog circuits 5a to 5n and measure a change in the phase of the calibration signal contained in the output signal of each analog circuit. There are two methods of doing this. According to one method, as shown in FIG. 20, signals extracted from the outputs of the analog circuits 5a to 5n of respective ones of the branches are selected one at a time by a switch 8, subjected to an analog-to-digital conversion by an AD converter 9 and fed back to the phase calibrator 7. According to the other method, as shown in FIG. 21, the outputs of the analog circuits 5a to 5n are combined by a combiner 10, after which the combined signal is converted to a digital signal by the AD converter 9 and fed back to the phase calibrator 7.
In FIGS. 20 and 21, the phase calibrator 7 includes the phase adjusters 7a1, to 7an for adjusting the transmit-signal phases of each of the branches; the phase controller 7b for generating the phase adjustment values, which are for applying a phase correction having characteristics opposite the phase characteristics of the analog circuits 5a to 5n, and inputting the corrected signals to the phase adjusters 7a1 to 7an; a calibration signal generator 7c for generating a calibration signal; adders 7d1, to 7dn for inserting the calibration signal into the transmit signals of respective ones of the branches; a phase estimation unit 7e for estimating the phase characteristics of each of the analog circuits 5a to 5n using the calibration signal that has been fed back; and a calibration controller 7f for controlling the overall phase calibrator.
FIG. 22 is a diagram useful in describing the timing at which the calibration signal is inserted into the transmit signals on each of the branches. A calibration signal SC is not inserted into transmit signals SM1 to SMn of the respective branches simultaneously but at different times. That is, the phase of a branch into which the calibration signal SC has been inserted is estimated, a phase adjustment is performed based upon the result and then this is repeated for each branch. In FIG. 22, the branches undergo the phase adjustment one after another in regular order. However, no problems arise even with an arrangement in which the phase adjustment is carried out collectively after the phases of all branches have been estimated.
With the scheme of FIG. 20 that employs switching, only a transmit signal SMi of a branch into which the calibration signal SC has been inserted, as well as this calibration signal SC, is fed back to the phase estimation unit 7e. With the scheme of FIG. 21 that employs combining, however, the transmit signals (SM1+SM2+ . . . SMn) on all branches and the calibration signals SC are fed back to the phase estimation unit 7e, as illustrated in FIG. 23.
FIG. 24 shows the details of the combining scheme. Here the calibration signal SC is spread by a spreading code in a spreader 7g and the spread signal is inserted in turn into transmit signals SMi of branches selected by a selector 7h. Further, a demodulated signal is despread in a despreader 11, the calibration signal is extracted and is input to the phase estimation unit 7e. Each of the analog circuits 5a to 5c is composed of a modulator MD and high-frequency amplifier HFA. Portions of the outputs of the analog circuits are extracted using directional couplers 12a to 12c, the extracted signals are input to the combiner 10, which has a hybrid construction, the combined signal from the combiner 10 is demodulated to a baseband signal by a demodulator 13 and the demodulated signal is input to the spreader 11 after undergoing an analog-to-digital conversion.
An adaptive array antenna system that performs calibration without halting system operation is known in addition to the prior art described above (see the specification of JP 2003-143047A). The latter prior-art system transmits a reference signal Y, extracts a signal component that is correlated with the reference signal from a receive signal and corrects transmission weighting coefficients based upon this signal component.
In the arrangement shown in FIG. 20, the branch signal fed back is selected by the switch 8. Insofar as calibration is performed, phase can be estimated by inputting only the transmit signal of a certain selected branch and the calibration signal that was inserted into this transmit signal to the phase estimation unit 7e. However, there is the possibility that phase deviations between ports (P1Q, P2Q, P3Q, . . . ) of the switch 8 will lead to calibration error. In particular, phase calibration requires that phase deviation be adjusted to a precision of several degrees or less. With the switching arrangement of FIG. 20, however, there are instances where the phase deviation between ports becomes tens of degrees, which is not desirable. Further, an undesirable property of the switch is that its phase characteristic fluctuates owing to environmental conditions such as temperature.
With the arrangement of FIG. 21 in which use is made of a signal obtained by combining the signals of each of the branches in the combiner 10, the problem of phase deviation is eliminated and calibration error can be suppressed. Specifically, a phase deviation between ports can lead to calibration error even when the combiner 10 is used. However, since the combiner 10 employs a hybrid of a microstrip line, for example, there are no mechanical and semiconductor portions and phase deviation is small in comparison with an active element such as a switch. Further, since the combiner (hybrid) 10 is a passive element, it can be considered highly reliable with regard to malfunction.
With the arrangement based upon the combiner 10 in FIG. 21, however, all transmit signals on all branches are combined and fed back. Consequently, in the ratio of the inserted calibration signal SC to the other transmit signals (SM1+SM2+ . . . SMn), as shown in FIG. 23, the calibration signal is small, the calibration signal, which is at the crux of phase estimation, has a poor S/N ratio and hence there is the likelihood of a decline in phase adjustment accuracy. More specifically, the larger the value given by the following equation, the greater the improvement in calibration:(calibration signal)/(transmit signal)=SC/(SM1+SM2+ . . . SMn)With the arrangement of FIG. 21, however, this value is small and a decline in phase estimation accuracy results.
With the scheme shown in FIG. 24, a calibration signal can be extracted by despreading. However, the transmit signal still remains as noise. In order to implement highly precise calibration of within a few degrees, it is necessary to lengthen the time for integrating the results of despreading, thereby improving calibration-signal extraction precision and raising calibration precision, as shown in FIG. 25. However, the problem is that estimation requires a long period of time. Furthermore, in addition to the long period of time needed to integrate the results of despreading, there is an increase in the scale of the circuitry owing to an increase in the number of bits of the integrator. It should be noted that if the noise level is low, the desired calibration precision can be obtained in a short period of time, as indicated by the solid line in FIG. 25.
Further, with the prior art illustrated in the specification of JP 2003-143047A, the implementation for correcting the transmit weighting coefficients branch by branch is complicated. In this example of the prior art, problems similar to those encountered with the switching and combining schemes of FIGS. 20 and 21 arise in a case where shared use is made of weighting coefficient/correction units.